Ultraprecise portable atomic clocks are on the verge of a breakthrough. An international team lead by scientists from the University of Zurich shows that it may be possible to use the latest generation of atomic clocks to resolve structures within the Earth.

An international team headed by astrophysicists Philippe Jetzer and Ruxandra Bondarescu from the University of Zurich is convinced that ultraprecise portable atomic clocks will make identifying mineral deposits or concealed water resources within the Earth using clocks a reality in the next decade. They say that such clocks will provide the most direct measurement of the geoid – the Earth’s true physical form. It will also be possible to combine atomic clocks measurements to existent geophysical methods to explore the interior of the Earth.

Today, the Earth’s geoid – the surface of constant gravitational potential that extends the mean sea level – can only be determined indirectly. On continents, the geoid can be calculated by tracking the altitude of satellites in orbit. Picking the right surface is a complicated, multivalued problem. The spatial resolution of the geoid computed this way is low – approximately 100 km.

An initial high-precision atomic clock prototype, ACES (Atomic Clock Ensemble in Space), is already due to be taken to the Columbus Space Lab at the International Space Station (ISS) by 2014. (picture: European Space Agency ESA, D. Ducros)

Using atomic clocks to determine the geoid is an idea based on general relativity that has been discussed for the past 30 years. Clocks located at different distances from a heavy body like our Earth tick at different rates. Similarly, the closer a clock is to a heavy underground structure the slower it ticks – a clock positioned over an iron ore will tick slower than one that sits above an empty cave. “In 2010 ultraprecise atomic clocks have measured the time difference between two clocks, one positioned 33 centimeters above the other,” explains Bondarescu before adding: “Local mapping of the geoid to an equivalent height of 1 centimeter with atomic clocks seems ambitions, but within the reach of atomic clock technology.”

According to Bondarescu, if an atomic clock is placed at sea level, i.e., at the exact altitude of the geoid, a second clock could be positioned anywhere on the continent as long as it is synchronized with the first clock. The connection between the clocks can be made with fiber optics cable or via telecommunication satellite provided that the transmission is reliable enough. The second clock will tick faster or slower, depending on whether it is above of beneath the geoid. The local measurement of the geoid can then be combined with other geophysical measurements such as those from gravimeters, which measure the acceleration of the gravitational field, to get a better idea of the underground structure.

“By 2022 at the earliest, one such ultraprecise portable atomic clock will fly into Space on board an ESA satellite,” says Professor Philippe Jetzer, the Swiss delegate for the STE-Quest satellite mission aimed at testing the general relativity theory very precisely. As early as 2014 or 2015, the “Atomic Clock Ensemble in Space ACES” is to be taken to the International Space Station ISS. ACES is an initial prototype that does not yet have the precision of STE-QUEST.